Digging without knowing where your drain pipes are is one of the most common — and costly — mistakes a homeowner can make. In New Zealand, underground drainage can sit anywhere from just a few centimetres below the surface to well over a metre deep, depending on your site, soil type, and when the house was built. Taking the time to locate and confirm pipe depths before you break ground can save you from a cracked sewer line, an expensive repair bill, and a potential health hazard.
Check Your Property's As-Built Drainage Plans First
The best starting point is your as-built drainage plans (sometimes called "as-builts"). These are drawings that show the layout and depth of your drainage system as it was actually installed. Here's where to find them:
- Your local council — Most councils hold as-built records for drainage connections. Contact your council's infrastructure or building services team and request the drainage as-builts for your property address.
- Your property file — Some drainage plans are lodged with the building consent records held by your council. You can usually request a property file search online or in person.
- Previous owners or builders — If your home is relatively new, the builder or developer may have copies.
Keep in mind that older properties (pre-1980s especially) may have incomplete or no records at all. Plans are also a guide — actual pipe positions can vary from what's drawn, so always verify on the ground.
Use a Pipe Locator or Cable Detector
If plans are unavailable or unclear, a pipe and cable locator (also called a cable avoidance tool or CAT) can help you trace the path of buried services. These devices detect signals from metallic pipes and cables. However, standard locators have limitations:
- They work well on metal pipes (cast iron, copper, galvanised steel) but struggle with PVC or clay pipes, which are very common in NZ drainage.
- For non-metallic pipes, a drain camera inspection or CCTV drain survey carried out by a licensed drainlayer is far more reliable.
- Some locators can pick up a signal from a sonde (a small transmitter) pushed through the pipe by a drainlayer, which then pinpoints the pipe's exact location and depth.
Hire or borrow a locator from Bunnings or Mitre 10, or engage a professional who has specialist equipment.
Look for Physical Clues on the Surface
Even without plans or equipment, you can gather useful information by looking around your property:
- Inspection chamber (IC) lids — These are the round or square access covers set into the ground. They mark the line of your drain and give you a direct way to measure depth. Simply open the lid and measure down to the invert (bottom) of the pipe.
- Downpipe and gully trap locations — Stormwater and wastewater pipes generally run in straight lines from these points toward the street or a soakage area.
- Ground depressions or lush grass strips — These can indicate a leaking or shallow pipe below.
- Boundary to house fall — NZ drainage is gravity-fed, so pipes slope downhill (typically at a minimum grade of 1:60 for 100 mm pipes under NZS 3604 and the NZ Building Code). Knowing the fall direction helps you predict depth changes across your section.
Dial Before You Dig — Contact Dial Before You Dig NZ
For public utility services (water mains, gas lines, telecoms, power cables) that may cross your property, contact Dial Before You Dig (0800 248 344 or online at beforeudig.co.nz). This free service notifies utility companies who then provide plans of their assets near your dig site. Note that this covers public infrastructure, not your private on-site drainage — but it's an essential step before any excavation.
Trial Holes and Hand Digging
Once you have a reasonable idea of where a pipe runs, the safest way to confirm exact depth is to carefully dig a trial hole by hand directly over the suspected pipe line. Use a spade carefully and switch to hand tools as you get deeper. This is slow but avoids the risk of a mechanical digger striking a pipe. A standard rule of thumb in the trade is to hand-dig the last 300–500 mm once you're in the vicinity of a known service.
Typical Drain Depths in NZ
| Drain Type | Typical Depth Range |
|---|---|
| Stormwater (near house) | 300–600 mm |
| Wastewater (near house) | 400–800 mm |
| Wastewater (near boundary) | 600 mm–1.2 m+ |
| Council connection point | 1.0 m–2.0 m+ |
These are general guides only — actual depths vary significantly by site. Always verify on your specific property.
When to Call a Professional
Some tasks related to locating and working on drains are restricted work under the Plumbers, Gasfitters and Drainlayers Act 2006. A licensed drainlayer must carry out any work that connects to, alters, or repairs your drainage system. If you're unsure of pipe locations, a licensed drainlayer can:
- Carry out a CCTV drain inspection to locate and assess pipes
- Use a sonde locator to pinpoint exact positions and depths
- Provide a professional report you can rely on before excavation
Never assume it's safe to dig near drainage without proper investigation — a broken sewer line is a health hazard and can be expensive to repair, and you may need a building consent depending on the scope of work (check with your local council). You can find qualified, local drainlayers on tradedirectory.co.nz.
Safety note: Always treat any underground service as live or pressurised until confirmed otherwise. Keep excavations safe and compliant with WorkSafe NZ guidelines, particularly for trenches deeper than 1.5 metres, which require specific shoring.